In recent years, as the demand for indoor voice communication and data communication has grown due to the spread of mobile phones, the development of a home-use base station installed indoors has been pursued. As a form of operation of such a home-use base station, a way of implementing communication in which only a pre-registered mobile phone(s) is connected to a home-use base station has been studied. Since a range covered by a home-use base station is considerably smaller than that of a base station installed outdoors, the range is called “femtocell”. Accordingly, a home-use base station is referred to as “femto base station” hereinafter.
Femto base stations as well as base stations in existing mobile communication networks transmit a common pilot signal. A mobile station performs synchronization establishment, channel estimation, and the like by receiving such a common pilot signal, and then performs data transmission/reception with a base station. Therefore, it is necessary to be able to receive a common pilot signal with appropriate receiving quality in a mobile station in order to provide appropriate communication quality.
In base stations in existing mobile communication networks, the transmission power of a common pilot signal to be transmitted in each cell is set in advance to a fixed value. In contrast to this, for common pilot signals transmitted by femto base stations in femtocells, a way of autonomously setting the transmission power by a femto base station has been studied. Patent document 1 (page 14, line 8 to page 15, line 21) discloses a method like this.
Its specific example is explained with reference to FIG. 8. Referring to FIG. 8, a macro base station 81 forms a macrocell 801 and transmits a common pilot signal CP1 with a constant transmission power to communicate with a mobile station (not shown). Femto base stations 812A and 812B form femtocells 802A and 802B respectively. Further, each of the femto base stations 812A and 812B measures a received power Pmacro [dBm] of the common pilot signal CP1 of the macro base station 811, and they transmit common pilot signals CP2A and CP2B respectively with a transmission power Pmacro+Poffset [dBm] by using the same radio frequency as that of the macro base station 811 to communicate with a mobile station (not shown). Note that Poffset is a constant value common to all the femtocells 802A and 802B.
Further, as data transmission from a base station to a mobile station on the downlink is speeded up, data transmission from the mobile station to the base station on the uplink is also speeded up. For the speedup on the uplink from a mobile station to a base station of these speeding-up schemes, the data transmission speed is maximized by maximizing the transmission power of the mobile station within the range in which the total amount of the received power (RTWP: Received Total Wideband Power) in the base station remains at or below a predetermined target value.
The femto base station like the one described above has been studied for use in systems such as WCDMA and E-UTRAN. In WCDMA, data transmission is performed by using an dedicated channel, of which transmission power is controlled, on an uplink and a downlink, or is performed by using a shared channel on a downlink as shown in Non-patent document 1. Further, in E-UTRAN, a radio frequency band is divided into a plurality of PRBs (Physical Resource Blocks) as shown in Non-patent document 2. A scheduler provided in an E-UTRAN base station assigns PRBs, and a base station performs data transmission with a mobile station by using an assigned PRB.
[Patent Document 1]
    UK Patent Application Publication No. 2428937 A[Non Patent Document 1]    3GPP TS 25.214 V7.3.0 (2006-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 7)[Non Patent Document 2]    3GPP TS 36.300 V8.1.0 (2007-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)